When really really strong demons have children, depending on the methods, one of them is always left weak after the birth. This is why it's encouraged and integrated into their instincts to procreate with strong partners, because after the birth it's not just the infant who needs help but also the birther who loses most of their magic/ki/energy in the process
This is where the strong partner comes into play, they are responsible for taking care of the infant for the time being as the other parent recuperates. Being demons it's already wired into their brains to be protective of what's theirs, now add in a baby, boom you got a cocktail of a very protective and powerful demon. Which leads to the emperor of all realms having an infant wrapped to his chest during all of its infancy.
Not only is it instinct, but also a show of power to show off your offspring to everyone and anyone knowing full well that no one could be a threat to them.
Meanwhile, the other parent is in bed rest for the majority of the child's infancy, since there is still a connection between them and the child, and the child continues to feed off of them, magic wise.
They slowly gain their powers as the child leaves infancy.
The stronger the child the longer the infancy stage lasts as their newborn body accommodates the large amount of magic flow within them.
This leaves the parent vulnerable, unless they have a strong partner with them.
Also, I know this might not be the shadowpeach kiss you all might have wanted but Magic CPR✨:
Because of the constant loss of magic via feeding the child, the mother is weak for a long period of time and to combat this the partner can share their energy with them (this only works if the partner has a stronger energy level than the other,, it'll do no good for both parents to be weak)
Not many magics are compatible with each other, so sharing magic is always tricky business, but it's possible between parents since one of them has been growing a life form with the other's magic and is used to it.